1. Field of the Invention
The invention relates to a method and apparatus for recovering a sterilizing gas, in particular ethylene oxide, in which materials to be sterilized are sterilized in a sterilizing chamber with a sterilizing gas. A gas flow is withdrawn from the sterilizing chamber and at least a part of the sterilizing gas contained therein is separated from the gas flow.
2. Description of the Prior Art
Usually, alkylene oxides, in particular ethylene oxide, are used as sterilizing gases which are generally mixed with gases which render them inert, such as Freon (dichlorodifluoromethane) or inert gases, in particular nitrogen. For costs and environmental reasons the sterilizing gas mixtures, i.e. the actual sterilizing gas and the other constituents rendering them inert, are not simply released into the environment after a sterilizing operation but are recovered and used again.
Thus, US-PS 3,549,312 discloses a method and an apparatus for recovering alkylene oxide and the admixed inert constituents in which the mixture of alkylene oxide and the inert additives extracted from a sterilizing chamber are supplied to a precooler and subsequently to an absorber. The mixture freed from moisture in the absorber then passes to a condenser for liquefaction of the alkylene oxide and the admixed Freon R 12 and thereafter to a reservoir for intermediate storage of the liquid mixture. This mixture can finally be supplied to the sterilizing chamber again from the reservoir subjected to excess pressure. A disadvantage with this recovery method is that the sterilizing chamber is pumped out after a sterilizing cycle in order to supply the gas or gas mixture to the connected recovery section of the apparatus and to be able to use it in the following sterilizing cycle. In a recovery method according to U.S. Pat. No. 3,549,312 during the pumping of the sterilizing gas or the gas mixture, simultaneously ambient air is allowed into the sterilizing chamber. Although the admitted air and the sterilizing gas or gas mixture to be pumped off have large density differences and thus remain separate from each other in a natural manner, there is a possibility, that in particular towards the end of the pumping operation to an increasing extent in and undesirable manner ambient air gets into the recovery apparatus. This danger is present precisely because the sterilizing chamber must be pumped down to small sterilizing gas residues in order to be able to open said chamber thereafter for the next sterilizing cycle.
According to a method disclosed in EP-A 0 130 319 for purifying ethylene oxide or a mixture of ethylene oxide and a fluorinated hydrocarbon, the sterilizing chamber subjected to excess pressure during the sterilizing is first relieved of pressure after each sterilizing process and thereafter evacuated with an oil-free operating vacuum pump down to a pressure of between 10 and 100 mbar. The discharged and thereafter extracted gases are purified and recovered in order to be made available for a new sterilizing operation. This remaining evacuation down to at least 100 mbar is intended to keep the loss of sterilizing gas down to an acceptable level. In an apparatus operating by this method, within the recovery section of the sterilizing apparatus, complicated pump and cooling systems are necessary for evacuating the sterilizing chamber. Furthermore, during the recovery itself, the pressure in the components and conduit systems of the recovery section changes continuously from the initial pressure in the chamber, lying above atmospheric pressure, down to the pressure of 10 to 100 mbar.
A method and an apparatus for recovery of a sterilizing gas, according to which after a sterilizing process the sterilizing gas is likewise pumped out of the sterilizing chamber and recovered in a recovery cycle is disclosed in German patent application no. 41 17 306.6, which was not previously published.
A further method and an apparatus for recovery of at least one component of a sterilizing gas is disclosed in EP-A-0 326 985. To discharge the gas mixture from the sterilizing chamber, it is proposed in the case of a chamber pressure lying close to atmospheric pressure to force a gas, for example nitrogen, at a higher pressure into the sterilizing chamber and thereby expel the sterilizing gas. The chamber pressure here will lie at least at times above the atmospheric pressure, thereby increasing the danger of leakage losses, i.e. escape of toxic sterilizing gas to the environment. Another disadvantage is the high nitrogen consumption on expelling the sterilizing gas.
Furthermore, it is known from EP-A-0 417 592, in the recovery of solvents collecting in continuously conducted production processes, for example on drying coated video and audio tapes, to fill the dryer with an inert gas and to recover the solvent vapours arising during drying together with said inert gas in a solvent recovery cycle and to return the gas freed from solvent vapours to the dryer again. In a removable apparatus for such a solvent recovery plant as apparent from German patent application P 40 40 389, which was not previously published, mixture can be withdrawn and simultaneously pure inert gas supplied to the dryer not only via the solvent cycle but also via an additional exit and inlet conduit. The objective of this is to enable the gas atmosphere of the dryer to be brought within a very short time to normal values again or maintained within the admissible values in the event of trouble in the dryer.